The invention relates to a double-sided adhesive tape strip and to its use for structural bonds which despite meeting a high profile of requirements are easy to separate again.
Adhesive tapes for structural bonds are known and are obtainable commercially. Bonds produced with them, however, are almost impossible to separate again without destroying or damaging the substrates. Adhesive tapes for reversible bonds are likewise known and obtainable commercially. In particular, adhesive tapes for rereleasable bonds, which can be released again by pulling in the direction of the bond plane, are known and are obtainable commercially under the designation xe2x80x9ctesa Power-Stripsxe2x80x9d. Bonds produced using such products offer a powerful hold and can be removed again without trace and without damaging the substrate or the adherends, as is described in DE 33 31 016 C2. The strength levels which can be achieved, however, are not adequate for structural bonds.
DE 3714453, DE 4222849, DE 4233604, DE 4339604, DE 4428587, DE 4431914 and DE 19511288 describe specific embodiments and applications of the abovementioned adhesive tapes. Stringent requirements are made of the adhesive tapes described in these documents; for instance:
In order to function flawlessly they must offer a tack which is sufficient for the particular application (for ease of bonding=low application pressure, and for immediate load bearing) and bond strength (during the period of the application).
Prerequisites for the detachment process are high extensibility coupled with low extensional stress and a tensile strength which is high in comparison with the detachment force (stripping force).
A marked reduction in tack on stretching is advantageous for the detachment process.
Appropriate ageing resistance in the bonded joint is essential for relatively long-term bonds.
Under high mechanical stresses (high shear and tip-shear loads), when using unsaturated styrene block copolymers (styrene, isoprene and styrene-butadiene block copolymers), ozone cracks may develop in the adhesive which may in turn cause the adhesive tapes to tear during the detachment process or may lead to the bonded article becoming detached during the period of application.
Especially in the case of bonds subject to high shear and tip-shear loads, high thermal shear strength is essential for sufficient bond strength at application temperatures of  greater than about 35xc2x0 C.
For many applications there is a requirement that the adhesive tape be pigmented. The use of pigments, such as TiO2, however, can have an adverse effect on the bond strength, especially when used in high concentrations. These adhesive tapes, however, are also inadequate for structural bonds.
Bonds which are rereleasable without destruction, using products in accordance with DE 33 31 016 (e.g. Power-Strips) produce strengths in the range of 1-2 kg (according to BDF testing: JO PMX 100, tip-shear durability test) and shear strengths of 30-40 N/cm2 (according to BDFxe2x80x94bond strength JO PM 0020). In order to reach the range of structural adhesive bonds which can be rereleased without destruction, higher values are required.
DE 33 31 016 C2, however, has also already disclosed adhesive tapes of the type described whose adhesive composition has been formulated so as to be heat-activatable. It has therefore disclosed products which, like a hot-melt, do not bond at room temperature but do bond under heat. With these products too it is impossible to obtain strengths sufficient for structural adhesives.
The object of the invention was therefore to attain the profile of requirements set out above (especially greater than 100 N/cm2 shear strength) while at the same time permitting easy detachment of the adhesive bond.
This object is achieved by an adhesive tape strip as defined more closely in the claims.
In particular, the use of reactive resin, preferably in combination with magnesium oxide, in combination with heat curing from the formulations of DE 33 31 016, produces a structural adhesive tape with a certain fixing aid, which is therefore particularly suitable for use in practice.
Particularly suitable bonds are those having spacers on at least one substrate, as is described in DE 196 37 223 A1. If bonding is carried out with 0.5 mm spacers, the assembly (e.g. metal/glass) lends itself particularly well to stripping on smooth surfaces; in other words, it can be separated easily and without residue or destruction.
Particularly suitable in accordance with the invention as reactive resins are alkylphenol resins which are in particular base-reactive, as marketed as polychloroprene-based adhesives, for example as Alresen(copyright) PA 565 (Hoechst).
The invention will be illustrated below using examples but without wishing unnecessarily to restrict it.